Phthalic anhydride is an intermediate compound used principally in the manufacture of plasticizers, polyesters and alkyd resins. The plasticizers are of two types: diesters of a monohydric alcohol, e.g., dibutyl phthalate, and mixed esters of two monohydric alcohols. Plasticizers can be made from the reaction of a variety of acids and/or anhydrides with an alcohol or a mixture of alcohols. It would be extremely desirable to find an inexpensive source of acids and/or anhydrides for use in forming plasticizers.
The present inventors have discovered a suitable source of acids and/or anhydrides for use in synthesizing plasticizer esters. This source is the phthalic anhydride residue from the various distillation or fractionation columns used in the finishing section of conventional phthalic anhydride processes. These distillation columns provide a readily available source of phthalic anhydride, benzoic acid, maleic anhydride, citraconic anhydride, etc. which can be reacted with an alcohol or a mixture of alcohols to form a mixture of esters.
Phthalic anhydride is typically produced from raw materials such as orthoxylene (o-xylene), petroleum naphthalene, and coal-tar naphthalene. The price of these raw materials and, as a direct result, the price of phthalic anhydride have fluctuated greatly depending upon supply and demand. Because the cost of the raw materials is a major factor in the price of phthalic anhydride it is of great importance that any system used to produce phthalic anhydride capture as much of the resultant product as possible.
Phthalic anhydride can be successfully produced from any of a number of processes, i.e., (1) air oxidation of o-xylene in fixed-bed reactors, (2) air oxidation of petroleum or coal tar naphthalene in fixed-bed reactors, (3) fluid bed oxidation of o-xylene, (4) fluid bed oxidation of petroleum or coal tar naphthalene, and (5) liquid phase oxidation of o-xylene.
The general process scheme for the various vapor phase routes is to mix the hydrocarbon feed (in the vapor form) with compressed air and to feed the mixture to fixed-bed reactors which contain tubes packed with catalysts, e.g., vanadium oxide and titanium dioxide coated on an inert, nonporous carrier. After the product stream exits either the fixed-bed or fluid bed reactors, it is cooled to cause the phthalic anhydride to condense. This allows separation of the phthalic anhydride from the gas stream. The crude phthalic anhydride is usually heat-treated in a decomposer, and in some cases chemical treatments are added to the decomposer. The purpose of the heat treatment is to dehydrate any phthalic acid in the crude to phthalic anhydride, to boil off materials such as water, and to form either condensation or volatile products from the various impurities so that the subsequent product purification by distillation is simplified. After distillation, the pure molten product may be solidified, flaked, bagged, and stored in a warehouse. Alternatively, the molten product may be pumped into large storage tanks and then into tank cars for shipment.
Distillation typically occurs in two fractionation columns or towers connected in series, wherein the bottoms stream from the first fractionation column is fed into the second fractionation column wherein substantially pure phthalic anhydride is taken overhead therefrom. The overhead stream from the first fractionation column is typically a light residue which comprises phthalic anhydride, maleic anhydride, benzoic acid and citraconic anhydride. The bottoms stream from the second fractionation column is typically a heavy residue (HOYS) which comprises phthalic anhydride and a mixture of compounds such as pryomellitic di-anhydride, anthraquinone, anthraquinone carboxylic acid, trimelletic acid or anhydride, biphthalyl, and various unknown components. Typically, the residue has an acidity per gram about equivalent to phthalic anhydride. Optionally, the bottoms from the second tower may be fed to a concentrating or residue drum where phthalic anhydride is recovered overhead and the distillation bottoms (i.e., heavies or HOYS) are concentrated into a residue stream.
The light residue from the first fractionation column and the heavy residue from the second fractionation column can, optionally, be disposed of by mixing together and then burning. This provides various environmental disposal problems. It also involves the wasting of commercially valuable products such as phthalic anhydride, maleic anhydride, benzoic acid, citraconic anhydride, and pryomellitic di-anhydride.
The present inventors have discovered a process which disposes the phthalic anhydride in an environmentally acceptable way and which avoids the increased cost associated with burning the residue or disposing by other means. The process of the present invention is capable of utilizing the light and heavy phthalic anhydride residue to synthesize mixed esters which are useful as low grade plasticizers, plasticizer extenders, lubricants, and/or greases. Thus, taking what are normally considered waste streams from the fractionation columns of the finishing section of a typical phthalic anhydride synthesis process and creating useful, saleable products by reaction of the streams with suitable alcohols in the presence of a catalyst.
The present invention also provides many additional advantages which shall become apparent as described below.